A method for adjusting the quantity of fuel supplied into a fuel collection line of a fuel-supply system for an internal combustion engine having direct fuel injection is described in published German patent document DE 195 39 885. Via a fuel line, a first, electrically-driven fuel pump supplies fuel from a fuel storage reservoir to a second high-pressure fuel pump, which is mechanically driven by the internal combustion engine. This second fuel pump in turn supplies the fuel to a plurality of fuel injectors via a fuel-collection line (rail). These fuel injectors inject the fuel directly into assigned combustion chambers.
The high-pressure fuel pump is mechanically coupled to a driven shaft of the internal combustion engine, which means the operating speed of the high-pressure fuel pump is proportional to the rotational speed of the driven shaft of the internal combustion engine, which rotational speed may differ considerably. The driven shaft may be a crankshaft or a camshaft of the internal combustion engine.
In order to be able to adjust the fuel quantity conveyed by the second fuel pump into the fuel collection line independently of the rotational speed of the internal combustion engine, an electromagnetic quantity-control valve is provided. Using the quantity control valve, a discharge side of the second fuel pump can be connected to a low-pressure side of the second fuel pump, in one switching position of the quantity control valve. In another switching position of the quantity-control valve, the connection between the discharge side and the low-pressure side is interrupted, in which case the second fuel pump pumps the fuel from its high-pressure side to the low-pressure side, i.e., no delivery into the fuel-collection line takes place.
Published German patent document DE 197 31 102 describes opening a switching valve, which is arranged in a similar manner as the previously mentioned quantity-control valve described in published German patent document DE 195 39 885, during overrun operation of the internal combustion engine. Thus, the high-pressure fuel pump does not supply fuel during overrun operation of the internal combustion engine.